Muskrats and evolution

StoryImage( ‘/Images/Story//Auto-img-111099110812959.jpg’, ‘Photo courtesy of’, ‘The muskrat is the most valuable fur-bearing animal in the United States, and some gourmands relish it as much hare or swamp rabbit.’);

The muskrat, Ondatare zibethicus,, is the most common fur-bearing animal in Illinois as well as the rest of the continental United States. Though its numbers are decreasing in our state due to the ruthless destruction of their wetland habitat, there is little danger of them becoming extinct because of their prolific rate of reproduction. Many a rural resident in Illinois and elsewhere routinely supplement their income by trapping these rodents, but the numbers of “rat” trappers has steadily declined in recent years because of the low price being paid for the pelts. Ronald Reagan, as a boy living in Dixon, Ill., is reported to have fished, hunted, and trapped muskrats along the Rock River, as did Jimmy Carter in his native Georgia.

The color of the fur in these interesting and economically important animals provides us with a fascinating study in how the process of evolution of species by natural selection works. Every trapper knows that muskrat fur varies in hue from a dark, (almost black) rich brown to a fulvous or reddish brown, but most do not know the percentages of colors in the same litter is dependent in large part upon the environment in which they were born.

A typical batch of muskrats born in a marsh will consist of two black mice and four reddish-brown ones. In a litter born in a drainage ditch or open field, one will find the blacks outnumber the browns by nine to one, with some litters consisting of all black.

These varying color ratios are explained by the laws of adaptive survival that apply to all living things. Coat color in the muskrat is determined by the interaction of many genes (the units of inheritance), and the genes that determine a brown coat are more prevalent in marsh-inhabiting rats than the ones resulting in black fur.

A muskrat in a marsh has ample cover with which to conceal itself from the searching eyes of predators, and the color of its fur has no survival value as far as protective coloration is concerned. Consequently, the frequencies of the genes for color remain the same from one generation to the next, and the 2 to 1 ratio of browns to blacks is maintained.

The situation changes drastically with muskrats living in open fields or along drainage ditches. These rats venture forth from their burrows in search of food and fodder only at night, and a black one is much more apt to escape the eyes of the owl or fox than is a lighter one. This means that each time a predator nabs a brown rat for dinner, he is selecting a black one to be a parent. Over a period of many generations, the frequency of genes for black will increase in the breeding population, and the determiners for brown will decrease to a point where almost all the rats are black.

This change in the characteristics of an animal in response to its environment and the natural selection of the best suited is one of the basic tenets of Darwin’s theory of the evolution of species.

Sometimes, as it is with the genes of all living species, the determiners change by a process known as mutation, and a different expression of the gene results. This happened in muskrats many generations ago, and now we infrequently encounter a white muskrat. This almost white animal is not an albino as a few dark hairs are scattered throughout its coat, and the eyes are pigmented.

A white rat has little chance of surviving much beyond its first one or two forages from the den, and only a very few will be lucky enough to escape a predator and grow to reproductive age. White is certainly a disadvantageous mutation and is selected against by nature. Consequently, the frequency of the white gene is always very low in a population, and few whites are produced. A white rat has literally blown its cover.

Some years ago, a former student of mine and part-time muskrat trapper presented me with a taxidermic mount of what is called a “Maryland White.” This unique specimen occupied a place of prominence in my office and was quite useful in instigating conversation with uncommunicative, shy visitors. All I would have to do to break the ice was to ask “Did you ever see a Maryland White?”

When I retired and moved to this area some years ago, I donated the white rat to the education department of the Salisbury, Md., Zoo. On a recent visit to Salisbury, I was delighted to learn the specimen was still in good condition and used effectively in the education program.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

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